This invention relates in general to detectors for multiple document feeds and in particular to detecting a phase shift in an ultrasonic signal.
Scanners and copiers use document feeders to transport documents into the machine. Mechanisms used for the transportation of documents, including paper or sheets of other material, have the capacity to accidentally pick up more than one document fed from a stack of documents. It is necessary to determine when more than one document is pulled into a document transport since multiple documents may jam the transport or prevent processing some documents.
There are two general methods for multiple document detection, contact and non-contact. The contact methods include measurement of small thickness changes with a contact foot or sensing arm that is in contact with the documents as they pass through the document transport. The contact foot is connected to a Linear Voltage Differential Transducer (LVDT), or a magnet, which is sensed by a Hall Effect Sensor. These sensors can detect changes in thickness of less than 1 xcexcm (10xe2x88x926 m).
The major disadvantage to the contact method is that anything in contact with moving paper, especially thin paper or ripped paper, can cause a malfunction such as a paper jam. The contact method also requires calibration using the maximum thickness document that will be fed through the document transport. When a thickness is measured which is above the calibration value plus a threshold, typically 30%, it is determined to be a multiple document feed. This method, however, will only work when documents having a uniform thickness are processed. Using a wheel on the end of the contact foot can reduce the chances of paper jam, however, the variations in the diameter of this wheel, due to the nonconformity in manufacturing, must be taken into account during the measurements.
The primary non-contact method for multiple document detection sends ultrasound signals through the document stream to determine if more than one document is present. Sending ultrasound through paper results in attenuation of the ultrasound signal. It is possible to determine the presence of multiple documents by change in attenuation of the signal received. This method is independent of the thickness of the individual documents and is made without making contact with these documents.
A typical contact document scanner is able to detect about 94% of the test multiple documents. An attenuation detector that was tested was only able to detect about 86% of these same test multiple documents, thus there is an opportunity for improvement using ultrasound detection.
For detecting multiple documents by attenuation methods, the performance improves as higher frequency transmitters and receivers are used. Unfortunately, the cost of these components also increases with frequency. There is also a limited range of paper thicknesses that can be properly detected. Therefore, the attenuation method alone is not suitable for multiple document detection.
The phase shift of ultrasound signal passing through documents has been used to detect multiple document feeds. See U.S. Pat. No. 4,066,969 which is herein incorporated by reference. Unfortunately, using phase shift is not reliable since multiple documents may cause phase shifts greater than 360 degrees. For detecting multiple documents by phase methods, the performance decreases at higher frequencies because the wavelength is shorter and the method becomes more sensitive to signal variations.
The present invention provides an improved method and apparatus for multiple document detection, which is both accurate and relatively inexpensive.
According to one aspect of the present invention a method for determining a digital phase in a signal comprises sampling a reference signal for a low going edge. If the low going edge is not detected the reference signal is sampled again. If low going edge is detected a counter is initialized. The reference signal is again sampled if a high going edge is not detected the reference signal is resampled until the high going edge is detected. When a high going edge is detected a counter is started. A resulting signal is then sampled if the level of the resulting signal is high the resulting signal is sampled until a low going edge is detected. If a low going edge is not detected sampling of the resulting signal continues. If a low going edge is detected sampling is continued until a high going edge is detected at which point the counter is stopped. The counter updates a register. When the resulting signal was sampled for a high level the resulting signal is sampled until a high going edge is determined at which point the register is stopped and converted to a phase difference. In one embodiment of the invention the reference signal being sampled is a driver for an ultrasonic transmitter and the resulting signal is an electrical signal representative of a received ultrasonic signal.
A method for determining a phase shift in a signal which is transmitted and received in accordance with another embodiment of the present invention requires a few steps. In this method, a counter is started when a first cross over location from a first-state-to-second-state is detected in the transmitted signal. The counter is stopped when a second cross over location in the received signal is detected which matches the first-state-to-second-state direction of the first cross over location in the transmitted signal. A counter value in the counter is then converted to obtain a phase difference between the transmitted and received signal.
A method for determining the presence of multiple documents in accordance with yet another embodiment of the present invention also requires a few steps. In this method, a signal is transmitted through a document feed comprising one or more documents. The signal is received after it has passed through the document feed. A counter is started when a first cross over location from a first-state-to-second-state is detected in the transmitted signal. The counter is stopped when a second cross over location in the received signal is detected which matches the first-state-to-second-state direction of the first cross over location in the transmitted signal. A counter value in the counter is converted to a phase difference which identifies the number of the documents in the document feed.
An apparatus for determining a phase shift in accordance with another embodiment of the present invention includes a signaling system which transmits and receives a signal, a counter, a detector, and a converter. The detector starts the counter when a first cross over location from a first-state-to-second-state is detected in the transmitted signal and stops the counter when a second cross over location in the received signal is detected which matches the first-state-to-second-state direction of the first cross over location in the transmitted signal. The converter converts a counter value in the counter to a phase difference between the transmitted and received signals.
An apparatus in accordance with yet another embodiment of the present invention includes a document processing system, a document transport system, a signaling system which transmits and receives a signal, a counter, a detector, and a converter. The document transport system supplies at least one document feed to the document processing system and the document feed comprises one or more documents. The detector starts the counter when a first cross over location from a first-state-to-second-state is detected in the transmitted signal and stops the counter when a second cross over location in the received signal is detected which matches the first-state-to-second-state direction of the first cross over location in the transmitted signal. The converter converts a counter value in the counter to a phase difference which identifies the number of the documents in the document feed.
An advantage of the present invention is that detection device makes no contact with the paper and is relatively independent of the paper thickness.
Another advantage of the present invention is that it will cause less document jams than direct contact methods.
Yet other advantages of this invention are: lower cost than traditional phase difference measurements methods; reduced susceptibility to noise as compared to traditional methods; and more accuracy and repeatability.